The tryptophan synthase bienzyme complex transfers indole between the α- and β-sites via a 25-30 Å long tunnel

The bacterial tryptophan synthase bienzyme complexes (with subunit composition alpha 2 beta 2) catalyze the last two steps in the biosynthesis of L-tryptophan. For L-tryptophan synthesis, indole, the common metabolite, must be transferred by some mechanism from the alpha-catalytic site to the beta-catalytic site. The X-ray structure of the Salmonella typhimurium tryptophan synthase shows the catalytic sites of each alpha-beta subunit pair are connected by a 25-30 A long tunnel [Hyde, C. C., Ahmed, S. A., Padlan, E. A., Miles, E. W., & Davies, D. R. (1988) J. Biol. Chem. 263, 17857-17871]. Since the S. typhimurium and Escherichia coli enzymes have nearly identical sequences, the E. coli enzyme must have a similar tunnel. Herein, rapid kinetic studies in combination with chemical probes that signal the bond formation step between indole (or nucleophilic indole analogues) and the alpha-aminoacrylate Schiff base intermediate, E(A-A), bound to the beta-site are used to investigate tunnel function in the E. coli enzyme. If the tunnel is the physical conduit for the transfer of indole from the alpha-site to the beta-site, then ligands that block the tunnel should also inhibit the rate at which indole and indole analogues from external solution react with E(A-A). We have found that when D,L-alpha-glycerol 3-phosphate (GP) is bound to the alpha-site, the rate of reaction of indole and nucleophilic indole analogues with E(A-A) is strongly inhibited. These compounds appear to gain access to the beta-site via the alpha-site and the tunnel, and this access is blocked by the binding of GP to the alpha-site. However, when small nucleophiles such as hydroxylamine, hydrazine, or N-methylhydroxylamine are substituted for indole, the rate of quinonoid formation is only slightly affected by the binding of GP. Furthermore, the reactions of L-serine and L-tryptophan with alpha 2 beta 2 show only small rate effects due to the binding of GP. From these experiments, we draw the following conclusions: (1) L-Serine and L-tryptophan gain access to the beta-site of alpha 2 beta 2 directly from solution. (2) The small effects of GP on the rates of the L-serine and L-tryptophan reactions are due to GP-mediated allosteric interactions between the alpha- and beta-sites.